The present application relates generally to combustion product control, and more particularly, to a method and system for use in simultaneous control of oxides of nitrogen (NOx) and ammonia (NH3) slip downstream of a selective catalytic reduction (SCR) system.
At least some known electric power generating facilities include combined cycle power plants that include one or more gas turbines, at least one heat recovery steam generator (HRSG), and at least one steam turbine. The HRSG and the steam turbine are coupled in flow communication through steam piping. The gas turbines and the HRSG are coupled in flow communication through combustion gas ducts. The gas turbines are operated to generate power, i.e., typically electric power. A combustion exhaust gas stream including waste heat in generated by the gas turbines is channeled to the HRSG to generate steam through the combustion gas ducts. The steam is channeled to the steam turbines to generate power, i.e., typically electric power.
Many known HRSGs include a selective catalytic reduction (SCR) system for removing regulated combustion products, e.g., nitrogen oxides (NOx) from the combustion exhaust gas stream prior to exhausting the gases to the atmosphere through an exhaust stack. Many known SCR systems include a bed of catalyst for removing at least some of the NOx from the exhaust gas stream. A reductant, such as ammonia (NH3), is injected into the exhaust gas stream entering the SCR system to facilitate further removal of NOx from the exhaust gas prior to entering the stack and then the atmosphere. Not all of the NH3 may be reacted with the exhaust gases and some of the unreacted NH3 passes through the SCR system and exits the exhaust system with the exhaust gas. Such unreacted NH3 is referred to as “ammonia slip.” Such ammonia slip typically becomes more prevalent during load transients of the combined cycle power systems with the accompanying exhaust gas transients, i.e., startups, shutdowns, and electric power generation ramps that approach and/or exceed certain power generation ramp parameters.
Many of the known SCR systems include a control architecture that includes a cascaded structure with an outer control loop for NH3 injection that includes a NOx measurement in the stack as the primary variable. The outer control loop establishes a NH3 injection flow rate setpoint. The cascaded control architecture also includes an inner control loop that includes NH3 slip as measured in the stack as a secondary variable. The inner loop for the NH3 slip facilitates maintaining the NH3 injection rate close to the setpoint generated by the outer control loop. Each of the outer loop and the inner loop includes a dedicated proportional-integral-differential (PID) controller. Each of the two PID controllers also receives NOx and NH3 measurements from the gas turbines and the stack through a continuous emissions monitoring system (CEMS). The NH3 injection flow rate is regulated to maintain measured NOx close to a predetermined stack NOx setpoint. Such regulation is accomplished fairly easily during steady-state operation of the combined cycle power system by establishing a substantially constant NH3 injection flow rate setpoint and regulating the flow to that setpoint. However, during significant transients, e.g., step changes in exhaust gas generation during startups and shutdowns, as well as steep load ramps during certain electric power dispatching situations, sequences of constant NH3 injection flow rates through a sequence of constant setpoints facilitates either one of NOx excursions and/or NH3 slip excursions. Typically, the control systems overcompensate with the NH3 injection flow rates to mitigate NOx excursions, thereby increasing NH3 slip excursions with the resultant excess costs of NH3 waste.